- Fix use of String operator + in runtime and standalone libraries.

runtime/bin/directory_impl.dart

Index: runtime/bin/directory_impl.dart
===================================================================
--- runtime/bin/directory_impl.dart	(revision 5311)
+++ runtime/bin/directory_impl.dart	(working copy)
@@ -82,8 +82,7 @@
         _path = result;
         callback();
       } else if (_onError != null) {
-        _onError("Could not create temporary directory [$_path]: " +
-                 "${result[1]}");
+        _onError("Could not create temporary directory [$_path]: ${result[1]}");
       }
     });
   }
@@ -95,7 +94,7 @@
       _path = result;
     } else {
       throw new DirectoryException(
-          "Could not create temporary directory [$_path]: " +
+          "Could not create temporary directory [$_path]: "
           "${status._errorMessage}",
           status._errorCode);
     }